1. Field of the Invention
The present invention relates to a method for transponder access control.
2. Description of the Background Art
Access control methods of this type are used, for example, in contactless identification systems or so-called Radio Frequency Identification (RFID) systems. This type of system typically includes a base station or a reader or a reader unit and a plurality of transponders or remote sensors, which are located simultaneously in the response range of the base station. The transponders or their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such inactive systems are called passive systems when they do not have their own power supply and semipassive systems when they have their own power supply. Passive transponders draw the energy necessary for their supply from the electromagnetic field emitted by the base station.
For data transmission from a transponder to a base station with UHF or microwaves in the far field of the base station, so-called backscatter coupling is employed. To that end, the base station emits electromagnetic carrier waves, which are modulated and reflected by the transmitting and receiving device of the transponder by a modulation method in accordance with the data to be transmitted to the base station. The typical modulation methods for this are amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is modified.
An access control method for transponders is described in the standards proposal ISO/IEC_CD 18000-6C of 7 Jan. 2005. The transponder in this case is first selected in a selection or arbitration process from a number of transponders. The described selection process is a stochastic process in the form of a slot-based ALOHA protocol. Selection methods of this kind are described in detail, for example, in the handbook by Klaus Finkenzeller, RFID-Handbuch [RFID Handbook], 3rd ed., HANSER, 2002.
When the transponder is chosen or selected or singled out, the reader unit transmits a request to the transponder in the form of a retransmission of a random number previously transmitted by the transponder within the scope of the arbitration process, whereupon the transponder transmits protocol control bits (PC) and an identification in the form of an electronic product code (EPC) to the reader unit. The protocol control bits contain information on a physical layer of the transmission path. The identification or the electronic product code EPC represents inter alia a product tagged with the transponder. The assignment of an EPC to the tagged product is standardized, so that the product can be inferred from the EPC information. Furthermore, the EPC can be used by the reader unit as a pointer to other information, for example, to a password assigned to the EPC. The password can then be used to block memory areas in the transponder for write accesses.
The transmission of the EPC upon request to the reader unit holds certain risks, however. Thus, during transport of products unauthorized persons can search selectively for products of a certain type, for example, high-quality watches or weapons, because the product-identifying EPC is transmitted unprotected. To accomplish this, it is only necessary to bring a suitable reader into the range of the transponders, for example, in a highway parking area or train stations.
After the transmission of the PC and EPC by the transponder, read and/or write access to memory areas within the transponder is possible through the reader unit, unless the specific areas are blocked or locked for write access. Furthermore, read access to passwords can also be barred. Despite a possible read blocking of passwords, there is some security risk that unauthorized persons have access to perhaps private memory contents.
When the transponder is used in so-called chip cards, personal data may be stored as memory contents. It is desirable here as well to regulate access to these data, for example, so that upon entry into a store it cannot be determined automatically by reading the memory contents whether the particular customer still has funds on the chip card.